Electrophotographic imaging processes and techniques have been extensively described in both the patent and other literature, for example, U.S. Pat. Nos. 2,221,776; 2,277,013; 2,297,691; 2,357,809; 2,551,582; 2,825,814; 2,833,648; 3,220,324; 3,220,831; 3,220,833 and many others. Generally, these processes have in common the steps of employing a photoconductive insulating element which is prepared to respond to imagewise exposure with electromagnetic radiation by forming a latent electrostatic charge image. A variety of subsequent operations, now well-known in the art, can then be employed to produce a permanent record of the image.
Various types of photoconductive insulating elements are known for use in electrophotographic imaging processes. In many conventional elements, the active components of the photoconductive insulating composition are contained in a single layer composition. This composition is typically placed in electrical contact affixed, with a conductive support during the electrophotographic imaging process.
Among the many different kinds of photoconductive compositions which may be employed in typical single active layer photoconductive elements are inorganic photoconductive materials such as vacuum evaporated selenium, particulate zinc oxide dispersed in a polymeric binder and homogeneous organic photoconductive compositions composed of an organic photoconductor solubilized in a polymeric binder, and the like.
The use of two or more active layers in a photoconductive element has been discussed in the patent literature. Such multi-active-layer photoconductive elements are sometimes referred to hereinafter simply as "multi-active" photoconductive elements. A partial listing of representative patents discussing or at least alluding to "multi-active" photoconductive elements includes: Hoesterey, U.S. Pat. Nos. 3,165,405; Bardeen, 3,041,166 issued June 26, 1962; Makino, 3,394,001 issued July 23, 1968; Makino et al., 3,679,405 issued July 25, 1972; Hayaski et al., 3,725,058 issued Apr. 3, 1973; Canadian Pat. No. 930,591 issued July 24, 1973; Canadian Pat. Nos. 932,197-199 issued Aug. 21, 1973; and British Pat. Nos. 1,343,671 and 1,337,228.
Although there has been a fairly extensive description of specific types of multi-active photoconductive elements in the literature, various shortcomings still exist in these elements so that there is a need to investigate alternative kinds of multi-active elements. For example, the multi-active elements described in the aforementioned Hoesterey patent suffer from the disadvantages of generally low speed and difficult to clean zinc oxide materials in both active layers of the element. Other multi-active elements such as those described in Canadian Pat. Nos. 930,591 and 932,199 appear to be primarily designed for use in a positive charging mode of operation and therefore may not generally be suitable for use in an electrophotographic process in which a negative charging mode is employed.
Co-pending Mey, U.S. Ser. No. 641,058 filed Dec. 15, 1975, discloses a multi-active photoconductive insulating element having at least two layers comprising an inorganic photoconductor-containing layer in electrical contact with an aggregate photoconductive layer. Copending Berwick et al., U.S. Ser. No. 639,039 filed Dec. 9, 1975, discloses a multi-active photoconductive insulating element having at least two layers comprising an aggregate or charge generation layer in electrical contact with an organic photoconductor-containing or charge-transport layer. The aggregate photoconductive layer of both Mey and Berwick et al., includes a continuous electrically insulating polymer phase having dispersed therein a finely divided, particulate co-crystalline complex containing at least one pyrylium-type dye salt and at least one polymer having an alkylidene diarylene group in a recurring unit.
The aggregate layer used in both Mey and Berwick et al. are of the type described in Light, U.S. Pat. No. 3,615,414. Typically, it has its principle absorption band for radiation in the visible region of the spectrum within the range of from about 520 nm to about 700 nm. Within this range the aggregate layer provides an exceptional level of sensitivity. However, below 520 nm, especially in the region of 460 nm, the aggregate layer exhibits low absorption thereby lowering the overall efficiency of such multi-active elements for white light exposure as well as decreasing the ability of such elements to discriminate red copy from a white background. Clearly, there exists need in the art for multi-active photoconductive elements comprising an aggregate photoconductive layer which has greater sensitivity in the region of the visible spectrum below about 520 nm especially in the blue region of the spectrum around 460 nm.